The judges' feedback said the model used to manage Whangawehi was unique, and should be considered a worldwide example, however the group formed too recently to base its achievements on decades of monitoring data.
The winner was the James River Initiative, an American-based project.
The other finalist was the Chicago River Restoration Project, another large-scale American initiative.
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The International Riverprize recognises and rewards organisations making waves in the sustainable management of the world's rivers.
Previous winners and finalists have received widespread recognition and built new partnerships.
Past winners include the River Thames in England, and the Danube River, a transnational river in Europe.
The Whangawehi Catchment Management group consists of marae, locals farms, forestry companies and other agencies who are working together to improve the health of the river and its tributaries.
Work includes fencing, retiring farm land bordering the river and planting trees.
Last year the group won the Asia-Pacific prize at the awards.